The cellular communication networks which have lately been established worldwide in many densely populated regions provide an unprecedented measure of individual accessibility in cooperation with already established land-line communications networks. The economic advantages for the consumers of corresponding services arising from this justify the high costs of the infrastructure connected with this. Under normal topographic conditions, a single fixed transmission station of such a network can cover mobile users at a distance of up to approximately 20 km, wherein the given maximum extent of a cell limited by this becomes considerably smaller in an unfavorable terrain. Based on positive experiences with communications satellites in geostationary orbits, the idea suggests itself to gain by similar means the considerable independence from the terrain achieved by this, as well as the large spatial extent of the covered area, also in connection with mobile communication systems.
While, because of the development of very low noise preamplifiers and mixers, radio reception from geostatic satellites became possible even with relatively small antennas, in connection with bidirectional linkages the problem of transmitting acceptable data rates over very large distances (approximately 36,000 km) by means of antennas transmitting almost non-directionally and at very low transmission output to the geostationary satellite remains. A solution of this problem lies in the use of satellites circling relatively low above the earth's surface, whose limited range is compensated by the presence of a multitude of identical satellites, which exchange information with each other and pass it on. Several concrete proposals already exist, among these are the IRIDIUM-concept (P. Brunt, “Iridium: Overview and Status”, in Space Communications, vol. 14, No. 2, 1996, pp. 61 to 68), and M-STAR and TELEDESIC (System Description Excerpt, Mar. 21, 1994).
A characteristic of all mentioned systems is the employment of a multitude of satellites moving in low orbits around the earth which, divided into sub-groups, are respectively evenly distributed over an orbit, which is distinguished in that it penetrates the plane containing the earth's equator at two points at an obtuse angle. The orbits of all satellites are arranged in respect to each other in such a way that an even coverage of the earth's surface by satellites is achieved. The orbits taken up by the satellites intercept each other in two points as a function of the inclination of the orbit. At the same altitude of all paths, a collision of satellites in different orbits is prevented by so-called inter-plane phasing and a selection of angles of the inclination of the individual orbital planes which is unlike 90°. At this time, the linkage of the individual satellites with each other takes place by means of appropriately aligned directional microwave antennas. This does not present a problem in connection with satellites in the same orbit, since the distance as well as the direction of the neighboring satellites are relatively stable. However, the situation becomes complicated with linkages with satellites of neighboring orbits.
In the course of one circumnavigation of the earth, a lateral change of the satellites flying along in neighboring orbits occurs at the intersections of all orbits. If there is a radio linkage with the satellites located laterally in respect to the direction of flight, it is necessary to perform tracking with the directional antenna over a larger angular spatial range as soon as the reception output falls below a minimum or, if this is not possible, a transfer to an antenna placed in a different direction must take place. The change of the directions of the neighboring satellites can take place almost instantaneously, if they are located nearly crosswise to the direction of flight. Therefore, under normal circumstances a transfer to another antenna takes place, because of which a contactless phase results because of the required acquisition time.
Also, rapid rotating movements of bodies (e.g., optical terminals) of large mass and spatial expanse, which are attached to the satellite body, add to the destabilization of the latter. Finally, the limited extent of the directional antennas results in the transmission of energy over a comparatively large spatial angle, even when using microwaves, because of which as many different transmission channels as possible must be available in view of a situation of high density of satellites occurring on account of the closeness of the intersecting points of all orbits. This forces a limitation of the bandwidth of the individual channels because of the limited bandwidth of the directional microwave antennas employed. However, this is unacceptable for the linkage of the satellites with each other (inter-satellite links), since information from other satellites is also passed on via these linkages, so that the flow of information has to be considerably higher than in the traffic between the ground and the satellites.
Today, many high bandwidth communication satellite networks are planned for future use. Most networks are intended to operate in a geostationary orbit (GEO). Other satellite networks are planned which will be operable in different kinds of orbits, i.e. in a low earth orbit (LEO), in a medium earth orbit (MEO), and in a high earth orbit (HEO). All networks aim at providing high bandwidth communication services for communication between satellites.
Satellite networks, as mentioned above, and other planned satellite networks offering high bandwidth communication services intend to use optical intersatellite links between different satellites, in order to establish a switching network in the space segment where the satellites are located. Such optical inter-satellite links are characterized in that they require high precision pointing methods and devices for the beam to be emitted, reaching over distances which may be as large as several thousands of kilometers, up to 80'000 kilometers, and allowing only extremely small angular deviations of the beam around a line of sight (LoS) remaining in the range of several micro-degrees.
In a common approach used to establish a coherent inter-satellite optical link, an electro-opto-mechanical multi-step acquisition procedure is applied. In this acquisition procedure, the laser beam to be received is first caught and then focussed on the receiver element. The acquisition procedure basically can be divided in two acquisition phases, a first phase (coarse acquisition phase) and a second phase (fine alignment phase). Usually, after completion of the fine alignment phase, the satellite terminals are ready for the transfer of nominal data (payload) and the communication phase is entered.
It is known in the art to use two laser beams with different laser wavelengths during the acquisition procedure, a first wavelength (L1) for coarse acquisition and a second wavelength (L2) for fine alignment. The first laser wavelength (L1) is received by direct detection means and the second laser wavelength (L2) is received by coherent detection means.
What matters is the use of two different detection schemes, direct detection and coherent detection, during the acquisition procedure. Direct detection stands usually for a wide field-of-regard. The coherent detection is applied to detect a laser beam that usually is narrow and therefore allows only for a much smaller field-of-regard (FoR). In this context, direct detection is understood to be a method of direct conversion from received optical power emitted by the terminal of a first satellite to an electrical position signal in the terminal of a partner satellite. The coherent detection is understood to represent a method that derives an electrical position signal in the partner satellite's terminal from the optical power received from the first satellite's terminal after superposition of the received optical power with a local oscillator laser in the partner terminal.
Generally, the transition between the two phases—the coarse acquisition phase and the fine alignment phase—represents a critical moment. Special attention has to be given to finding laser beam at the beginning of the fine alignment phase where coherent detection is starting in a field-of-regard that is about one order of magnitude smaller than the field-of-regard of the direct detection sensor at the end of the coarse acquisition phase. Additionally, it has to be considered that the terminals of both satellites involved, the emitting terminal as well as the receiving or partner terminal, are in rest but are exposed to micro-vibrations due to mechanical disturbances coupled in from the host satellite mechanical interface. These micro-vibrations are also called satellite-induced vibrations. Vibrations and other disturbances are referred to as transient impacts.